Abstract: Histological and molecular responses of grass carp Ctenopharyngodon idella to high concentration ammonia exposure were studied in this study by determining the lethal concentration, gill cellular structure, and expression patterns of sod and hsp90 genes. Grass carps were treated at five ammonia concentrations (50, 72, 104, 151 and 220 mg/L) for 24h, and then the gill tissues were collected for histological analysis. In addition, three tissues (gill, intestine and liver) were collected to measure the expression of sod and hsp90 genes. After exposure to high concentration of ammonia, the arrangement and structures of gill cells changed dramatically. Edema and fusion of secondary lamellae were observed, and the edge of cell and nucleus could not be clearly defined. Meanwhile, the two genes (sod and hsp90) in different tissues were unregulated significantly, indicating that high concentration of ammonia could impair the cellular structures and induce the expression of stress proteins. These results also suggested that sod and hsp90 were suitable biomarkers for ammonia exposure assessment in grass carp.